What more speed with a jet drive? In contrast, conventional propellers stop at 60%-72% efficiency. Smaller, lighter, cheaper jet engines continue to be developed. 2021 Jeanneau Leader 12.5 WA. The logic for airlines is that a jet engine can fly a plane faster between destinations, and thus the aircraft will burn around the same as a slower prop plane. Why would the prop need nearly twice the torque to so the same job? Michel1, you seem to have fallen into the common logic trap that high speed requires high efficency and that cost effectiveness requires high efficency. By Jeff Hemmel. The turboprop uses the power of a jet engine to turn a propeller. Ok, now the jet boat has twice the range or carrying capacity, which makes it the better choice as the propboat cannot meet the transport capacity needed. Jet drives aren’t compatible with every type of waterway. it is more effective to absorb the power developed in the jet engine by gearing it to a propeller instead of using it directly in the form of a pure jet stream. This makes them perfect for city airports that have residences nearby (Porter Airways, who exclusively flies turboprops has long pushed for a jet replacement fleet on the basis that they are quieter). This topic can be as deep as an ocean, and to approach it in the simplest way possible, we will need to avoid any in-depth technical knowledge that might be beyond those without aerospace engineering degrees. Raising the boat transom approximately six inches takes longer, depending on how you do it. The safety and space savings of the jet … The modern jet designs have the motors sit in the lower and further back creating more room in the boat. What?...now you want constant displacement? Jet Boat Options: Jet Drive or Sterndrive . Let us know in the comments. Neither are the case. Because of their unusual-ness in the broader aviation world, many pusher designs are relatively famous. 2. They range from the Piaggio Avanti and Beechcraft Starship to the large flying wing bombers produced by Northrop in the late 40’s and early 50’s such as the YB-35. Fuel efficiency, on the other hand, would again be directly related to the drive efficiency multiplied by the engine efficiency. Therefore, for example, if you replace the prop lower unit of a 100 hp engine with a jet unit, it will be putting out at most 75 hp at … Why so? More Boats. A good example of this is the recoil felt on the shoulder when firing a rifle, or the thrust felt when holding a powerful firehose. Turboprop: Efficiency . Jet aircraft, on the other hand, don’t have a traditional prop but rather a fan blade inside the engine casing. 2021 Reflex Reefrunner. The propeller can also be a hazard to swimmers behind the boat. A jet drive replaces the low-hanging propeller and gearcase to make these outboards specialists in navigating shallows and water strewn with debris and or obstacles. Yet, a 500 hp V-drive will typically do 90 or more mph, while the same jet will do just over 70. Turboprops should be considered to be jet engines that drive a propeller using the exhaust gasses. What do you think? It works in relation to Newton’s Third Law of Motion – “every action has an equal and opposite reaction”. Pump efficiencies around 90% or more are regularly attainable. I know there are negatives: parts availability, weight, fuel economy, complexity. Jet engines also have some advantages, such as quieter operation. jet drive crafts will run in less than 6 inches of water. ), Brake power required for Water jet propulsion, Converting Yamaha Traditional Prop to Jet Drive. Discussion in 'Jet Drives' started by Michael1, Oct 6, 2007. Slip factor on a v-drive cruiser size boat can be 20-30%. The compact four-cylinder engine that powers most jet drive systems takes up less space in the boat and is also lighter than a sterndrive engine, which can make the boat easier to tow on a trailer. This results in shorter rolls. Pumps work differently than propellers; they don’t show the same limits. A modern turboprop engine produces incredible power to weight compared to earlier era engines, while staying exceptionally light. We don't have a good explanation for this beyond the fact that the sterndrive boat weighs 600 pounds more than the outboard, and the jet drive system is less efficient than the outboard's propeller. High propulsive efficiency at low speeds, which falls off rapidly as airspeed increases. This means that regional airports that might not even have a concrete runway (especially in developing areas of the world), are perfectly safe to land on with a turboprop. At 20 knots, the former has a theoretical efficiency of 65%, against 52% of the propeller. Videos. By far, the most common jet drives are outboard jets. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. In addition if you ever need to stop quickly, then in theory a jet drive is the best means to do so. Boats. Other pod drive models push the boat in traditional fashion, with propellers mounted behind the drive shaft unit. A turboprop engine has a propeller that you would find on the first aircraft; however, the similarities to those piston engines end there. The prop issue doesn't worry me so much as my daughter has been on a prop boat her whole life and knows the rules. Oh, whats that?...You want constant HP...ok, then a jet drive is faster because you can use a lightweight GT to have a vessel with 1/2 the displacement of the prop boat has to carry around a couple of large chunks of iron. OOOHHHH....you want everything (weight, HP, torque, SFC, fuel type, etc). Other examples include: Burt Rutan’s Long EZ and its variants, the Convair B36, the Cessna 337 (a push-pull configuration), the Velocity family of homebuilts, a wide variety of military unmanned aircraft, and perhaps the strangest looking of the bunch… The performance of a 360hp prop boat over jet is quite the difference too. Propellers and piston engines are getting quieter and even more efficient. Be in the know. The first was that the cross-over point—where a pump jet becomes more efficient than a propeller—was unknown. Let’s examine. Another combination of jet and prop is the turboprop engine. Dirt strip bush planes will never see a jet engine. If we go back to my V-drive vs. Jet example, according to the diagram, the V-drive should be slower, because the conventional propeller prop efficiency shows a sharp decline after 20 knots, and crossing over with the jet drive at 50 knots. HJ 241 hamilton jet best suited engine option. Honda Marine Introduces Three New Jet-Drive Outboard Models. What exactly is the difference, and which is the better engine choice for aircraft? Boats. I looked at the diagram (I had seen it before, too), and I still don't know why efficiency and speed aren't related. If I have 500 hp, and the drives have two different efficiencies (including underwater drag of the drive system), then I would expect the system with the greater efficiency to have higher overall net thrust, and a higher top speed. Some research has been done on engines that marry both advantages together, but none has yet reached the market. When it comes to aircraft engines, there are two choices – a turboprop or a jet. This engine is able to develop very high thrust at low air speeds because the propeller can accelerate large quantities of air at zero forward velocity of the airplane. To prop or not to prop, that is the question when looking for a jet boat. I also question that diagram, or perhaps I'm not reading it correctly. With no gearcase or propeller extending below the hull, a jet drive allows you to maneuver around or pass over obstructions that would limit the travel of a conventional propeller-driven outboard. I know in larger prop boats the efficiency of a large single is superior to 2 smaller drives. Conversely a jet engine is perfect for longer ranges and has more power for a bigger aircraft with more passengers. The propeller in front of the engine becomes less efficient with faster-moving air. The efficiency of a jet is far poorer than a prop. How a Waterjet Works A waterjet generates propulsive thrust from the reaction created when water is forced in a rearward direction. Also, while the jet's appendage drag is less, since no gear case is being dragged through the water, this may result in increased wetted surface of the hull, since it offers no ability to trim the boat. If you plan to switch between prop and jet, depending on water conditions, winter, summer, etc., purchase a second cooling water pump so that each, the prop and jet drive, have one installed and you don’t have to transfer the pump back and forth. http://www.boatdesign.net/forums/showthread.php?t=17003, http://www.ultradynamics.com/sections/educational/why.asp, http://www.ultradynamics.com/sections/control/index.aspoffer, http://boatdesign.net/forums/showthread.php?t=11254, http://www.yachtforums.com/forums/technical-discussion/4470-jet-drive-vs-prop.html, (You must log in or sign up to reply here. Boats. Speed and efficency aren't related because you can always just throw more horsepower and less weight at the problem (i.e. Propellers and pump-jets are among the most important and sometimes sensitive components of a submarine class. More complicated and heavier than turbojet. A sterndrive also requires more maintenance than a jet drive. And with electronic multi-point fuel injection, you get lots of responsive power with fuel-sipping efficiency. A jet engine also allows a plane to fly higher and at greater speeds, although it burns far more fuel to do so. ... Piston vs. Large interior space: It has a large amount of space in the cockpit. Jets work best on hard bottoms such as rocky riverbeds. The outdrive has considerably more torque and HP at the prop than the jet has at the impeller (jet is showing ~ a 15% loss in brake HP). Based in Sydney, Australia. A waterjet is essentially a pump inside a very short pipe. If the pump jet was actually more efficient at plausible higher transit speeds, on balance that may offer a benefit over the entire mission profile, even if the pump jet … Go to 600 hp. We use ads to keep our content free. We will never see a propeller driven aircraft fly as fast or fly as high as a jet. Turboprop engines fly at speeds 20 to 30% lower than that of turbojets in the same category. 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